1. Technical Field
This invention relates to cordless telephones and more particularly to a cordless telephone having a plurality of portable units arranged for communicating with a base unit in a frequency hopping system.
2. Description of the Prior Art
The Federal Communications Commission (FCC) recently promulgated rulings in the utilization of spread spectrum systems, including a frequency hopping system. These rulings now allow suppliers to produce improved cordless telephones which provide users much greater freedom and mobility than is available with conventional cordless telephones. Frequency hopping systems spread their energy by changing, or hopping the center frequency of the transmission many times a second in accordance with a pseudo-randomly generated list of communication channels. The result is a significantly higher signal to noise ratio than may be achieved by conventional techniques such as amplitude modulation that uses no bandwidth spreading.
These improved cordless telephones provide an increased operating range over what was previously available. They also provide security naturally from an eavesdropper listening in on a conversation simply because of the spread spectrum/frequency hopping transmission technique employed by these telephones. Examples of such improved cordless telephones are described in U.S. Pat. No. 5,323,447 which issued to M. E. Gillis et al. on Jun. 21, 1994, U.S. Pat. No. 5,353,341 which issued to M. E. Gillis et al. on Oct. 4, 1994 and U.S. Pat. No. 5,463,659 which issued to W. J. Nealon et al. on Oct. 31, 1995.
Although security is provided naturally in these cordless telephones from would be eavesdroppers listening in on conversations, the spread spectrum/frequency hopping transmission technique employed by these telephones also can make difficult the attempts of a legiminate user to obtain access from one of the units in these telephones to another. Since a communication link between units is maintained only during actual communications by users of the telephones, such link has to be reestablished before each use. It is therefore desirable that communication links between cordless telephone units that operating in a frequency hopping system be quickly easily and established.